CH 5 etc - MUX A strength?

No expert but It could be that ....

Ch 5 is on mux A and mux A is 64QAM (like Mux 2 which has ITV/Ch4), unlike the BBC/Crown channels on Mux1,B,C and D are 16QAM which was shown to improve reception

You don't say which transmitter you are pointing at, some mux's are different strengths - click on the map and look at the ERP values in W

Chris

There are sometimes short or even long periods of reduced power from transmitters to allow work to be done on the mast, but that would affect all channels. Also, wherever possible, full power is restored at night. You could try calling BBC Engineering Information and asking.

Since all the multiplexes are in the same range of analogue channels, I would not immediately suspect the antenna - had one been on a vastly different channel number (and they can be) then the antenna would be suspect.

It could be that the signal level is marginal or there is multipath reception or other effect such as antenna position that is causing the signal conditions on channel 32 to be sufficiently worse to prevent the receiver from decoding the digital signal.

You don't say which transmitter you are using, but if it is one with an analogue TV channel on channel 33 or perhaps 31, then that may also be influencing channel 32.

Remember that although we are talking about digital TV, the actual RF that you are picking up is analogue and subject to the effects of that. With analogue TV, bad things appear as grain or patterning on the screen, whereas a poor analogue signal from which digital TV is derived will result in intermittent stopping and starting of the picture, blocks on the screen or it just not working. Added to this, the digital MUX transmitters are run at low power because they are often in immediately adjacent channels to analogue ones. All of this adds up to a less than ideal arrangement to get a good DTTV result.

There are a few things that I think you could try:

- Get an antenna installer to check the signal on each of the MUXes. This involves a spectrum analyser and/or an error rate tester and will confirm or deny whether there is a signal problem from the antenna. I tmay simply be that the signal is marginal from the current antenna.

- Try another antenna or move the current one around a little. Given that the reception is variable with channels etc., this may work or you may create a situation where other muxes disappear. Without test gear, you may get into a frustrating iterative process taking a lot of time and not getting a satisfactory result.

- Try another receiver. I don't think I'd do this without checking the antenna first - the issue is more likely to be from there. A newer receiver might be better at decoding from the analogue signal and give an OK result, but I wouldn't bet on it.

.andy

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Chris wrote in news: snipped-for-privacy@4ax.com:

Thanks Andy and Chris for the references; the links were the sort of thing I was trying to find, and very useful; and the modulation may have a bearing. there are a couple of possibilities, my MUX A has BBC2 next door on CH33 - it's Crystal Palace.

I have my doubts about the aerial. the guy who fitted it was reccommended, but he seemed to know less about it than I do, and he lacked the sort of test gear I would expect, but it appeared to be a workmanlike job, and worked at the time!

So I guess I'll have to find a real installer.

Still while I'm waiting, the Goodmans box with DAB may come along

mike r

In article , mike ring

One other thing to cause problems is kinking or overtight bends in the cable. This causes a "suck out" effect which only affects a narrow frequency range.

Andy Luckman wrote in news: snipped-for-privacy@house.local.net:

Thanks for the tip, Andy, but I don't think it's that; both TVs downstream of the amplifier are the same, and upstream it looks ok.

I fear it's time to open the wallet, as I'm not messing with it myself, (first get on the roof, and then stretch up the chimbley - I've only got one floor, but my bum goes two bob/half a crown just at the thought!}

mike r

Amplifier? They can also create funny effects if they are of poor quality. Did you try connecting the decoder directly to the antenna?

I think that that makes good sense. If you do get somebody in, do make sure that proper alignment and error checking is done and see it working with your decoder. This may imply not going for the cheapest deal.

.andy

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Andy Hall wrote in news: snipped-for-privacy@4ax.com:

I think it's ok - Labgear CM6011, and direct connection would be a bugger, I'd have to get a 25" telly up there as well

Certainly will this time

mike

A 6011 is a passive 2 way splitter. Could be a problem. Just disconnect and bypass it to test.

Really though, I think as you have said, get a "man" in to do it properly and certify the result. Best would be a CAI "man".

In article , mike ring writes

Be wary of using amplifiers as if your analogue signals are quite strong then you can get odd intermodulation products which result in spurious signals being splattered across the band, and some of this may be ending up on a digital carrier you want to receive!....

Andy Luckman wrote in news: snipped-for-privacy@house.local.net:

D'oh yes, sorry; it follows a masthead amplifier, what I was mixing it up with.

But it's a requirement that the downlead is split, so I'll still have to get it sorted; I'm sure I could d-i-y but I haven't the bottle to swan around up there, me and my moggie have no safety net, literally as well as metaphorically.

I'll look out for CAI

mike r

Take the cable that comes straight from the antenna, no amplifiers, splitters, video recorders, other boxes or anything in the way, and connect it to the decoder. Connect the decoder only by scart to the TV.

See what results you get then. If it still doesn't work you have a problem with the antenna or the cable or interference from an adjancent channel (or unwanted pickup maybe from a neighbour's VCR - try tuning your analogue TV to channel 32 and see if there's a faint picture there).

If it does work then the problem is in the kit you have between your antenna and the decoder.